This invention relates to synchronous motors fabricated using superconducting materials and, more particularly, to such a motor in which lines of flux are trapped within the rotor portion of the motor. Sections of the motor are intermittently recharged to periodically regenerate the lines of flux and maintain motor performance.
Trapping flux in a superconductor coil or in a bulk material can produce a relatively high magnetic field As the flux trapping capability of high T.sub.c superconducting (HTSC) materials improves, the need for fabricating high current conductors for motor applications will diminish, resulting in increased motor applications. One advantage with these new type motors is their substantially lower weight and smaller size for a given power rating as compared to conventional motors.
Theoretically, magnets made of HTSC materials will have five-to-ten times higher energy than magnets currently in use, for example, those made of rare earth. HTSC coils, or the bulk material used in the new motors, will need to be "charged" or magnetized just as conventional permanent magnets must be. To charge, a rotor comprised of the HTSC material is cooled from a temperature which is above a critical temperature level (4 degrees Kelvin (K), for example), to a temperature which is below this level. A magnetic field is applied to the rotor while it is being cooled, and this field is removed once the rotor temperature falls below critical level. The rotor, in order to maintain the field, induces circulating currents in the superconting material, and these trap flux in the rotor.
Motors which can be produced in accordance with this method are described in co-pending application 679,747, which is assigned to the same assignee as the present application, and is incorporated herein by reference. As described therein, a motor has a rotor of superconductive material and a stator whose windings can either be of a conventional electrically conductive material, or also of a superconductive material To achieve rotor magnetization, the rotor and stator are placed in either the same, or separate, cryostats, depending upon the type of motor (radial gap or axial gap) involved. Regardless, after rotor cooling and stator winding de-energization, the rotor then acts much like a permanent magnet, albeit one having a much higher level of magnetization than conventional or rare earth magnets. Further, the rotor can be magnetized to one level for one application, and to a different level for another application. The motor will remain operational so long as the rotor temperature is kept below the critical level.
A major problem with the above described motors is flux creep, or the dissipation of the trapped lines of flux over time. There are two problems which must be resolved in order to make use of superconductive motors of the type described above practical. First, it is important to be able to restore the lines of flux on some type of periodic basis so the motor will remain a functioning motor. Otherwise, the operating time of the motor (the time it takes for the flux to dissipate to the point where the motor will no longer operate under load conditions) will be so short that use of the motor will be impractical. Second, the method by which flux restoration is achieved must be as non-intrusive as possible. That is, the mechanism by which restoration is achieved should not so interfere with the normal operation of the motor that it cannot function under load conditions.